Test access architecture for multi-die circuits

ABSTRACT

Aspects of the invention relate to test access architecture for stacked dies. The disclosed test access interface for a die can function as a stand-alone test access interface, allowing both pre-bond testing and post-bond testing of the die. In a stack of dies, the test access interface of a die may be enabled/disabled by the test access interface of an adjacent die.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application No. 61/698,482, filed on Sep. 7, 2012, and naming Ronald Press et al. as inventors, which application is incorporated entirely herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of integrated circuit (IC) testing technology. Various implementations of the invention may be particularly useful for testing stacked integrated circuits.

BACKGROUND OF THE INVENTION

Three-dimensional integrated circuits (3-D ICs) have been shown to reduce on-chip global interconnect lengths and thus alleviate delay and power consumption problems. 3-D ICs can also facilitate the integration of dissimilar technologies (digital, analog, radio-frequency circuits, et al.). In a 3-D IC, multiple active layers or dies are vertically stacked on top of each other and are interconnected using “short” vertical links. These “short” vertical links are referred to as through-silicon vias (TSVs).

While 3-D ICs provide many benefits, such benefits can be realized only when major challenges in the areas of design, manufacture and test are addressed. In traditional IC testing, wafers are probed and individual dies tested before they are packaged. For 3-D ICs, both pre-bond testing and post-bond testing of wafers are needed to increase the yield. An important aspect for the post-bond testing is testing of TSVs because TSVs are key components of a 3-D IC, providing power, clock and functional signals. Even a single TSV defect between any two layers of a 3-D IC chip stack can void the entire chip stack, reducing the overall yield. Capability of testing interconnections between stacked dies should be included in any test architecture in a circuit design.

Another important aspect for the post-bond testing is test access. One or more test interface ports must be established through the bottom die to allow test of any die in the stack and of the interconnections between the dies. Only a limited number of TSVs, however, can be reserved for use by the testing circuitry. This is because the total number of TSVs on a die is limited due to the need for a “keep-out” area and most of these TSVs are required for power, clock, and signal lines.

It is thus desirable to design a test access architecture that provides efficient test access and control through a small number of dedicated TSVs for both modular die testing and interconnection testing.

BRIEF SUMMARY OF THE INVENTION

Aspects of the invention relate to test access architecture for stacked dies.

In a first aspect, there is a die comprising test circuitry for testing the die and/or for testing, when the die is stacked, interconnections between the die and an adjacent die. The test circuitry comprises, at a first side of the die, a first input port configured to receive test stimuli and test control instructions and a first output port configured to send test responses, test stimuli and test control instructions, there being a data signal path within the die between the first input port and the first output port. The test circuitry also comprises, at a second side of the die, a second input port configured to receive test responses, test stimuli, and test control instructions from another die and a second output port configured to send test stimuli and test control instructions to the another die, there being a data signal path within the die between the second input port and the first output port and a data signal path within the die between the first input port and the second output port. Further, the test circuitry also comprises test access interface circuitry that accepts data from either the first input port or the second input port depending on an input selection signal.

The test access interface circuitry may be compatible with IEEE 1149.1. The test access interface circuitry may be enabled/disabled by test access interface circuitry of an adjacent die connected to the first side of the die. The input selection signal may be derived from a test control instruction in the test access interface circuitry.

A die according to one aspect may further comprise one or more third input ports at the first side of the die configured to receive one or more control signals for the test access interface circuitry and one or more third output ports at the second side of the die configured to send the one or more control signals to the another die.

There may be a device on a signal path between one of the one or more third input ports and a corresponding one in the one or more third output ports, which is configured to block/unblock the signal path according to an enable signal. The blocking/unblocking of the signal path can disable/enable test access interface circuitry of the another die.

The enable signal may be the same signal as the input selection signal. The test access interface circuitry may be compatible with IEEE 1149.1, and the one of the one or more third input ports may be configured to receive a control signal of Test Mode Select (TMS) or a control signal of Test Reset (TRST).

In a further aspect, there is a method for testing a stack of dies comprising a first die and a second die stacked on top of the first die, the method comprising: enabling test access interface circuitry of the first die; sending a test control instruction to test access interface circuitry of the first die to enable test access interface circuitry of the second die; and loading test control instructions using a signal path passing through first an instruction register in the test access interface circuitry of the second die and then an instruction register in the test access interface circuitry of the first die.

A method according to one aspect may further comprises loading test stimuli using a signal path passing through boundary scan cells in the second die and the first die in sequence to test interconnections between the first die and the second die.

In a method according to one aspect, the stack of dies further comprises a third die stacked on top of the second die and the method further comprises: enabling test access interface circuitry of the third die by applying the test control instruction loaded into the instruction register in the test access interface circuitry of the second die; and loading test control instructions using a signal path passing through an instruction register in the test access interface circuitry of the third die, the instruction register in the test access interface circuitry of the second die and the instruction register in the test access interface circuitry of the first die in sequence.

In yet another aspect, there is a method for designing a die testable in a stack of dies, the method comprising: receiving design data of a die; modifying the design data by adding, at a first side of the die, a first input port for receiving test stimuli and test control instructions and a first output port for sending test responses, test stimuli and test control instructions; modifying the design data by providing a data signal path within the die between the first input port and the first output port; modifying the design data by adding, at a second side of the die, a second input port for receiving test responses, test stimuli, and test control instructions from another die and a second output port for sending test stimuli and test control instructions to the another die; modifying the design data by providing a data signal path within the die between the second input port and the first output port and a data signal path within the die between the first input port and the second output port; and modifying the design data by adding test access interface circuitry that accepts data from either the first input port or the second input port depending on a test control instruction signal.

Certain inventive aspects are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

Certain objects and advantages of various inventive aspects have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic of the test access architecture of IEEE Boundary Scan Standard 1149.1 (JTAG) (prior art).

FIG. 2 illustrates an example of a JTAG chain of three devices (prior art).

FIG. 3 illustrates an example of test access architecture for stacked dies based on JTAG-compatible test access interfaces according to various embodiments of the invention.

FIG. 4 illustrates an example of a schematic diagram for the daisy-chained JTAG-compatible test access interfaces shown in FIG. 3.

FIG. 5 illustrates a flow chart 500 describing a method of testing stacked dies that may be employed by various embodiments of the invention.

FIG. 6 illustrates a flow chart 600 describing a method of designing a die testable in a stack of dies that may be employed by various embodiments of the invention.

FIG. 7 illustrates a programmable computer system with which various embodiments of the invention may be employed.

FIG. 8 illustrates a schematic of a bi-direction boundary scan cell and a waveform diagram of contactless leakage testing.

DETAILED DESCRIPTION OF THE INVENTION

Various aspects of the present invention relate to test access architecture for stacked dies. In the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention may be practiced without the use of these specific details. In other instances, well-known features have not been described in details to avoid obscuring the present invention.

Some of the techniques described herein can be implemented in software instructions stored on a non-transitory computer-readable medium, software instructions executed on a processor, or some combination of both. As used herein, the term “non-transitory computer-readable medium” refers to computer-readable medium that are capable of storing data for future retrieval, and not propagating electro-magnetic waves. The non-transitory computer-readable medium may be, for example, a magnetic storage device, an optical storage device, a “punched” surface type device, or a solid state storage device. Some of the disclosed techniques, for example, can be implemented as part of an electronic design automation (EDA) tool. Such methods can be executed on a single computer or on networked computers.

Although the operations of the disclosed methods are described in a particular sequential order for convenient presentation, it should be understood that this manner of description encompasses rearrangements, unless a particular ordering is required by specific language set forth below. For example, operations described sequentially may in some cases be rearranged or performed concurrently. Moreover, for the sake of simplicity, the disclosed flow charts and block diagrams typically do not show the various ways in which particular methods can be used in conjunction with other methods.

Additionally, the detailed description sometimes uses terms like “send”, “receive”, and “load” to describe the disclosed methods. Such terms are high-level abstractions of the actual operations that are performed. The actual operations that correspond to these terms will vary depending on the particular implementation and are readily discernible by one of ordinary skill in the art.

Also, as used herein, the term “design” is intended to encompass data describing an entire integrated circuit device. This term also is intended to encompass a smaller group of data describing one or more components of an entire device, however, such as a portion of an integrated circuit device. Still further, the term “design” also is intended to encompass data describing more than one microdevice, such as data to be used to form multiple microdevices on a single wafer.

The present disclosure also includes some hardware drawings. These drawings are only schematic and are non-limiting. For illustrative purposes, the size of some of the elements in the drawings may be exaggerated and not drawn on scale, and some elements in the drawings may be omitted.

The hardware description sometimes uses terms like “first”, “second”, “third” and the like to distinguish between similar elements and not necessarily for describing a sequential or chronological order. The terms are interchangeable under appropriate circumstances and the embodiments of the invention can operate in other sequences than described or illustrated herein. Similarly, the terms “top”, “bottom”, “over”, “under” and the like in the description are used for descriptive purposes and not necessarily for describing relative positions. The terms so used are interchangeable under appropriate circumstances and the embodiments of the invention described herein can operate in other orientations than described or illustrated herein.

The test access architecture according to embodiments of the invention is based on a modular test approach, as will be discussed in detail later. The test access interface of a die in a stack according to embodiments of the invention may be made compatible with an industry standard such as IEEE Boundary Scan Standard 1149.1, allowing the die to have a stand-alone test access interface for both pre-bonding and post-bonding tests. Moreover, the test access interface of any die in a stack according to embodiments of the invention may be deselected (disabled). Test mode activity or functional mode activity, initiated when the test access interface is selected (enabled), can remain active even after the test access interface is deselected. Before discussing embodiments of the invention, the test access architecture of IEEE Boundary Scan Standard 1149.1 is briefly described.

FIG. 1 illustrates a schematic of the test access architecture of IEEE Boundary Scan Standard 1149.1, also known as JTAG. The TAP controller 110 is a 16-state finite state machine (FSM) that controls the JTAG engine based on control signals and instructions supplied through the TAP (test access port). The TAP has four pins that receive control signals and test data/instructions for driving the circuit blocks and controlling specific operations: TCK (JTAG Test Clock) 120, TMS (Test Mode Select) 130, TDI (Test Data Input) 140 and TDO (Test Data Output) 150. An optional pin, TRST (Test Reset), is not depicted in FIG. 1. At the rising edge of the TCK signal, the TMS signal determines the TAP controller state sequence. Keeping the TMS signal stable can prevent the TAP controller from changing its state. The TDI pin 140 and the TDO pin 150 are the serial data/instruction input and the serial data/instruction output, respectively. One bit of data is transferred in from the TDI pin 140, and out to the TDO pin 150 per rising clock edge of the TCK signal. The TRST control signal, when low, resets the test logic by forcing the TAP controller 110 to enter a reset state.

The instruction register 160 is a shift register-based circuit and is serially loaded with instructions. After a new instruction is shifted into the instruction register 160, the previous instruction remains valid until the TAP controller 110 changes to the state of updating instruction register. The TAP controller 110 can also reset, access the instruction register 160, or access data selected by the instruction register 160. The state of the TAP controller 110 and contents of the instruction register 160 determine which register (the boundary scan register 170, the instruction register 160, the bypass register 180, the ID register 190 or any other registers) is fed by the TDI pin 140 for any operation.

FIG. 2 illustrates an example of a JTAG chain. The TCK signal and the TMS signal, broadcasted through the pins 210 and 220, respectively, are shared by the three devices 230, 240 and 250. The TDO pin of the device 230 is connected to the TDI pin of the device 240 and the TDO pin of the device 240 is connected to the TDI pin of the device 250. The system TDI pin 260 and the system TDO pin 270 are connected to the TDI pin of the device 230 and the TDO pin of the device 250, respectively. Different instructions may be shifted into different devices such that they can work in different modes. For example, the devices 230 and 240 are configured so that their interconnections are tested while the device 250 is configured to perform a device test.

Embodiments of the invention include dies comprising test circuitry for testing the die and/or for testing, when the die is stacked, interconnections between the die and an adjacent die. The test circuitry comprises, at a first side of the die, a first input port configured to receive test stimuli and test control instructions and a first output port configured to send test responses, test stimuli and test control instructions, there being a data signal path within the die between the first input port and the first output port. The test circuitry also comprises, at a second side of the die, a second input port configured to receive test responses, test stimuli, and test control instructions from another die and a second output port configured to send test stimuli and test control instructions to the another die, there being a data signal path within the die between the second input port and the first output port and a data signal path within the die between the first input port and the second output port. Further, the test circuitry also comprises test access interface circuitry that accepts data from either the first input port or the second input port depending on an input selection signal.

FIG. 3 illustrates an example of test access architecture for stacked dies based on JTAG-compatible test access interfaces according to various embodiments of the invention. Three stacked dies (310, 320 and 330) are shown in the figure. Core logic of each die (313, 323 and 333) is connected with its neighbor(s) through TSVs, of which functionality can be overridden by boundary scan cells (315, 325 and 335). One type of the boundary scan cells that may be employed by various embodiments of the invention is bi-direction boundary scan cells as illustrated in FIG. 8. The use of bi-direction boundary scan cells enables contactless leakage testing, as discussed in a paper by Sunter et al., “Complete, contactless I/O testing reaching the boundary in minimizing digital IC testing cost,” Proceedings of International Test Conference, 446-455, 2002, which is incorporated herein by reference.

For testing, each die (at least for the dies 310 and 320) has an input port and an output port on each side of the die (referred to herein after as top and bottom sides, respective). At the bottom side, the input port is configured to receive test stimuli and test control instructions while the output port is configured to send test responses. At the top side, the input port configured to receive test responses, test stimuli, and test control instructions from another die while the output die is configured to send test stimuli and test control instructions to the another die. The test stimuli may be used to test interconnections between dies or core logic. The test responses are generated from the corresponding tests. The test control instructions are stored in instruction registers and used by the test access interfaces to control the test access architecture.

Each die also has a JTAG-compatible test access interface (318, 328 and 338), daisy-chained through the above input/output ports. These test access interfaces provide access and control for testing cores (BIST 340 and/or compression logic 350) and interconnections (Bndry Scan 360). FIG. 4 illustrates an example of a schematic diagram for these daisy-chained JTAG-compatible test access interfaces. In addition to the input and output ports (e.g., ports 410, 413, 420 and 423 for die 320) for data/instructions (TDI and TDO) discussed above, two or three input ports and corresponding output ports for control signals of TCK, TMS and optionally TRST are located at the bottom side and the top side, respectively (e.g., ports 415, 417/419, 425 and 427/429 for die 320).

As can be seen in FIG. 4, for each of the test access interfaces, there are a data signal path within the die between the input port for data/instructions at the bottom side and the output port for data/instructions at the bottom side (e.g., between ports 410 and 413), a data signal path within the die between the input port for data/instructions at the top side and the output port for data/instructions at the bottom side (e.g., between ports 420 and 413), and a data signal path within the die between the input port for data/instructions at the bottom side and the output port for data/instructions at the top side (e.g., between ports 410 and 423).

The test access interface for each die accepts data from either the input port for data/instructions at the bottom side or the input port for data/instructions at the top side based on an input selection signal. In FIG. 4, the input selection is controlled by a multiplexer (430, 440 and 450 for the test access interfaces 318, 328 and 338, respectively). The multiplexer is controlled by a next_tap_en signal that serves as the input selection signal) generated by the corresponding test access interfaces.

As also can be seen in FIG. 4, for each of the test access interfaces, there is a data signal path for each of the control signals within the die between the corresponding input port at the bottom side and the corresponding output port at the top side (e.g., between ports 415 and 425). An AND gate is inserted into the signal path for the control signal of TMS (gates 435, 445 and 455 for the test access interfaces 318, 328 and 338, respectively). By blocking/unblocking the signal path of TMS, the test access interface of the adjacent die above the die where the signal path is located can be deactivated/activated. The blocking/unblocking may be controlled by the same signal used for selecting inputs for the test access interface. In FIG. 4, for example, the next_tap_en signal is used to control both the multiplexer 440 and the AND gate 445 for the test access interface 328. This next_tap_en signal may be derived from a test control instruction in the test access interface. A bit in the test control instruction, for example, may be used for the next_tap_en signal.

Different implementations of enabling/disabling the test access interface of the adjacent die may be adopted. For example, Instead of blocking/unblocking the control signal of TMS, the control signal of TRST may be used to select/deselect (enabling/disabling) the test access interface of the adjacent die. As the deselecting also resets the test access interface, it cannot continue performing whatever test mode it may have been previously configured for.

FIG. 5 illustrates a flow chart 500 describing a method of testing stacked dies that may be employed by various embodiments of the invention. For ease of understanding, the flowchart 500 will be described with reference to the test access architecture illustrated in FIG. 3 and FIG. 4. It should be appreciated, however, that alternate implementations of a test access architecture may be used to perform the method of testing stacked dies shown in the flow chart 500 according to various embodiments of the invention. In addition, it should be appreciated that implementations of the test access architecture in FIG. 3 or the test access interface in FIG. 4 may be employed with other methods of testing stacked dies according to different embodiments of the invention.

Initially, in operation 510, the test access interface 318 of the first die 310 is enabled. The first die 310 may be the bottom die of the stack. In this case, the interface 318 is automatically enabled when the control signals are applied to the stack because all of the control signals are directly applied to the interface 318. If the first die 310 has an adjacent die below, the interface 318 can be enabled by the adjacent die below similar to operation 520.

Next, in operation 520, a test control instruction is sent to the test access interface 318 to enable the test access interface 328 of the second die 320. A bit of the test control instruction may be used to generate a next_tap_en signal shown in FIG. 4. Once the test control instruction is shifted into the instruction register of the interface 318 and is applied (activated), the AND gate 435 will allow the control signal to reach and thus enable the test access interface 328. In the meantime, the test access interface 318 will accept data from the TDO port 413 of the test access interface 328, rather than from its own TDI port 470 through the multiplexer 430.

Next, in operation 530, test control instructions are loaded using a signal path passing through first the instruction register in the test access interface 328 and then the instruction register in the test access interface 318. The operation 530 may be used to prepare the two test access interfaces for subsequent tests. For example, to test interconnections between the two dies 310 and 320, test stimuli are loaded using a signal path passing through boundary scan cells in the second die and the first die in sequence. Test responses are then captured and shifted out using the same signal path.

The flowchart 500 may be expanded if the stack of dies contains more than two dies. The test access interface of the third die 330 may be enabled by applying the test control instruction loaded into the instruction register in the test access interface of the die 320 in operation 520. Next, another set of test control instructions may be loaded using a signal path passing through the instruction register in the test access interface of the third die 330, the instruction register in the test access interface of the second die 320 and the instruction register in the test access interface of the first die 310 in sequence. This process may be repeated if there are more dies.

To test the interconnections between the dies 330 and 320 and between the dies 320 and 310, test stimuli may be loaded by using a signal path passing through boundary scan cells in the third die 330, the second die 320 and the first die 310 in sequence. The captured test responses can be shifted out following the same signal path.

FIG. 6 illustrates a flow chart 600 describing a method of designing a die testable in a stack of dies that may be employed by various embodiments of the invention. The method may include operation 610, receiving design data of a die. Next, in operation 620, the method may further include modifying the design data by adding, at a first side of the die, a first input port for receiving test stimuli and test control instructions and a first output port for sending test responses, test stimuli and test control instructions. Next, in operation 630, the method may further include modifying the design data by providing a data signal path within the die between the first input port and the first output port. Next, in operation 640, the method may further include modifying the design data by adding, at a second side of the die, a second input port for receiving test responses, test stimuli, and test control instructions from another die and a second output port for sending test stimuli and test control instructions to the another die. Next, in operation 650, the method may further include modifying the design data by providing a data signal path within the die between the second input port and the first output port and a data signal path within the die between the first input port and the second output port. Next, in operation 660, the method may further include modifying the design data by adding test access interface circuitry that accepts data from either the first input port or the second input port depending on a test control instruction signal.

The methods of designing a die testable in a stack of dies according to various embodiments of the invention may be implemented by executing programming instructions on one or more programmable computers/computer systems, such as the computing system discussed later. Correspondingly, some other embodiments of the invention may be implemented by software instructions, stored on a non-transitory computer-readable medium, for instructing one or more programmable computers/computer systems to perform the methods of designing a die testable in a stack of dies.

Various embodiments of the invention may be implemented through the execution of software instructions by a computing device, such as a programmable computer. FIG. 7 shows an illustrative example of such a programmable computer (a computing device 1001). As seen in this figure, the computing device 1001 includes a computing unit 1003 with a processing unit 1005 and a system memory 1007. The processing unit 1005 may be any type of programmable electronic device for executing software instructions, but will conventionally be a microprocessor. The system memory 1007 may include both a read-only memory (ROM) 1009 and a random access memory (RAM) 1011. As will be appreciated by those of ordinary skill in the art, both the read-only memory (ROM) 1009 and the random access memory (RAM) 1011 may store software instructions for execution by the processing unit 1005.

The processing unit 1005 and the system memory 1007 are connected, either directly or indirectly, through a bus 1013 or alternate communication structure, to one or more peripheral devices. For example, the processing unit 1005 or the system memory 1007 may be directly or indirectly connected to one or more additional memory storage devices, such as a “hard” magnetic disk drive 1015, a removable magnetic disk drive 1017, an optical disk drive 1019, or a flash memory card 1021. The processing unit 1005 and the system memory 1007 also may be directly or indirectly connected to one or more input devices 1023 and one or more output devices 1025. The input devices 1023 may include, for example, a keyboard, a pointing device (such as a mouse, touchpad, stylus, trackball, or joystick), a scanner, a camera, and a microphone. The output devices 1025 may include, for example, a monitor display, a printer and speakers. With various examples of the computer 1001, one or more of the peripheral devices 1015-1025 may be internally housed with the computing unit 1003. Alternately, one or more of the peripheral devices 1015-1025 may be external to the housing for the computing unit 1003 and connected to the bus 1013 through, for example, a Universal Serial Bus (USB) connection.

With some implementations, the computing unit 1003 may be directly or indirectly connected to one or more network interfaces 1027 for communicating with other devices making up a network. The network interface 1027 translates data and control signals from the computing unit 1003 into network messages according to one or more communication protocols, such as the transmission control protocol (TCP) and the Internet protocol (IP). Also, the interface 1027 may employ any suitable connection agent (or combination of agents) for connecting to a network, including, for example, a wireless transceiver, a modem, or an Ethernet connection. Such network interfaces and protocols are well known in the art, and thus will not be discussed here in more detail.

It should be appreciated that the computer 1001 is illustrated as an example only, and it not intended to be limiting. Various embodiments of the invention may be implemented using one or more computing devices that include the components of the computer 1001 illustrated in FIG. 7, which include only a subset of the components illustrated in FIG. 7, or which include an alternate combination of components, including components that are not shown in FIG. 7. For example, various embodiments of the invention may be implemented using a multi-processor computer, a plurality of single and/or multiprocessor computers arranged into a network, or some combination of both.

While the invention has been described with respect to specific examples including presently preferred modes of carrying out the invention, those skilled in the art will appreciate that there are numerous variations and permutations of the above described systems and techniques that fall within the spirit and scope of the invention as set forth in the appended claims. For example, while the test access interfaces compatible with the IEEE 1149.1 standard is used to describe the invention, it should be appreciated that various examples of the invention may be implemented using other types of test access interfaces such as those compatible with the IEEE 1500 standard. 

What is claimed is:
 1. A die comprising test circuitry for testing the die, interconnections between the die and an adjacent die when the die is stacked, or both the die and interconnections, the test circuitry comprising: a first input port configured to receive test stimuli and test control instructions; a first output port configured to send test responses, test stimuli and test control instructions, the first input port and the first output port being located at a first side of the die, wherein there is a data signal path within the die between the first input port and the first output port; a second input port configured to receive test responses, test stimuli, and test control instructions from another die, wherein there is a data signal path within the die between the second input port and the first output port; a second output port configured to send test stimuli and test control instructions to the another die, the second input port and the second output port being located at a second side of the die, wherein there is a data signal path within the die between the first input port and the second output port; a third input port configured to receive one or more test control signals; and test control circuitry, the test control circuitry accepting the one or more test control signals and producing a block/unblock signal derived from the one or more test control signals, the test control circuitry accepting data from either the first input port or the second input port depending on the block/unblock signal.
 2. The die recited in claim 1, wherein the test control circuitry can be enabled/disabled by test control circuitry of an adjacent die connected to the first side of the die.
 3. The die recited in claim 1, wherein the test control circuitry is compatible with IEEE 1149.1.
 4. The die recited in claim 1, wherein the test circuitry further comprises: one or more third output ports configured to send the one or more control signals to the another die, the one or more third output port being located at the second side of the die.
 5. The die recited in claim 4, wherein the test circuitry further comprises a device on a signal path between one of the one or more third input ports and a corresponding one in the one or more third output ports, the device being configured to receive both the block/unblock signal path and the one or more test control signals from the one or more third input ports, the device being further configured to selectively block or unblock the one or more test control signals from passing to the one or more third output ports and enabling test control circuitry of the another die according to the block/unblock signal.
 6. The die recited in claim 5, wherein the device is an AND gate.
 7. The die recited in claim 5, wherein the test control circuitry is compatible with IEEE 1149.1, and the one of the one or more third input ports is configured to receive a control signal of Test Mode Select (TMS) or a control signal of Test Reset (TRST).
 8. A stack comprising at least a first die and a second die both according to claim
 1. 9. A stack comprising at least a first die and a second die both according to claim 1, wherein the second output port of the first die is coupled to the first input port of the second die, and the second input port of the first die is coupled to the first output port of the second die.
 10. A method, comprising: receiving design data of a die; and modifying the design data of the die, the modifying comprising: adding a first input port for receiving test stimuli and test control instructions and a first output port for sending test responses, test stimuli and test control instructions, the first input port and the first output port being located at a first side of the die, providing a data signal path within the die between the first input port and the first output port, adding a second input port for receiving test responses, test stimuli, and test control instructions from another die and a second output port for sending test stimuli and test control instructions to the another die, the second input port and the second output port being located at a second side of the die, providing a data signal path within the die between the second input port and the first output port and a data signal path within the die between the first input port and the second output port, adding a third input port configured to receive one or more test control signals; and adding test control circuitry, the test control circuitry accepting the one or more test control signals and producing a block/unblock signal derived from the one or more test control signals, the test control circuitry accepting data from either the first input port or the second input port depending on the block/unblock signal.
 11. A non-transitory computer-readable medium having stored thereon instructions which, when executed on one or more processors, causes the one or more processors to perform a method of claim
 10. 